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Figure 1 shows the ratio of counts from a 14 second exposure of Omega
Cen in August, 2000 to the counts in a 100 second exposure, vs. the Y
position on the chip (WF2 in this case with gain=15). The raw values
(filled circles) fall below a ratio of 0.14 due to CTE loss (i.e., the
count rate and background are both lower for the short exposure,
leading to more CTE loss). The different panels are for different
target brightness, as described by the labels. The filled squares show
the values corrected using the Whitmore, Heyer, and Casertano formula
while the filled triangles show the values corrected using the Dolphin
formula. Note that the Dolphin formula is somewhat better for the top 4
panels, but is much worse for the faintest stars in the bottom panel.

Figure 2 shows a calculation based on the Whitmore, Heyer, Casertano
(1999) correction formula, assuming a very low background for the raw
image (i.e., 0.1 electron, appropriate for a very short exposure, a
narrow-band exposure, or an exposure in the UV) versus an exposure
which has been preflashed with 25 electrons. The ratio of the S/N for
the preflashed image versus the raw image is plotted vs the Log of the
target brightness. The S/N estimates include the uncertainties in the CTE
corrections. The three curves show the effects for a star near
the bottom of the chip (i.e., X = 400, Y = 100, where the preflash is
never an advantage since CTE loss is low and the preflash adds noise),
near the center of the chip, and near the top of the chip (where the
preflash is an advantage for the brighter targets).